Current commercial plastic materials such as polyethylene terephthalate (PET), polycarbonate (PC), polyethersulfone (PES), polyimide (PI) and the like each of which has advantages of a low cost and light resistance but also has disadvantages such as poor thermal stability or poor absorbent, thus the applications of such plastic materials in electronic engineering are limited.
With respect to these plastic materials, poly(arylene ether) (PAE) is a high performance engineering plastic having good physical properties, such as: 5% weight loss temperature (Td5%) at 493° C., high glass transition temperature (Tg) at 223° C., high plasticity, high chemical resistance, and good thermal stability, but its water uptake, water-oxygen penetration, the degree of crystallization, the transmittance of visible light area, and other features still require appropriate improvements. Polyethers are generally polymerized by dihalo monomer or dinitro monomer with bisphenoxides to form a polymer. In addition, sulfone or ketone groups have good nucleophilic replacement activity to aromatic halides, so that (poly(aryl ether sulfone)) or (poly(aryl ether ketone)) is common, and heterocyclic and a compound with amide groups can also be used as a polymerization active group to synthesize a PAE with high molar mass.
PAE plastic material is used mostly in insulating film and membrane gas separation, a fluorine-containing polymer is especially useful to provide a low dielectric constant and low water uptake, in which the six-fluorine groups on the side chain of the polymer also help to increase the solubility of the polymer (generally called the fluorine effect), and the huge group (CF3−) with electronegativity will increase the free volume of the polymer, thus improving physical and chemical properties of the polymer, such as gas permeability and electrical insulation, etc.
Further, in order to achieve a higher thermal stability, the design of the molecule adopts a less polar phenyl unit for substitution; it has a high conversion rate and a high molecular weight, and the glass transition temperature is about 250˜280° C. A polymer containing a pyridine and thiophene has a glass transition temperature about 70° C.˜80° C. lower than the average glass transition temperature of a polymer formed by a common monomer. This is due to the different catenation angles. The other influence on the glass transition temperature is the structure of bisphenol. In general, a polymer containing a huge fluorine group has a higher glass transition temperature; a polymer containing a softer group, such as bisphenol-A (BPA), has a relatively low glass transition temperature.
Currently, a traditional synthesis method of PAE comprises a step of proceeding a polymerization reaction of a glycol monomer having steric structure with a phenyl unit with low polarization, such as dihalo or dinitro monomer polyether, wherein the step has advantages for forming cross-linked network of the polymer during the nucleophilic displacement reaction to improve the thermal stability, reduce dielectric constant and water uptake. Therefore, due to the chemical and physical properties of PAE plastics, a PAE with steric structure is a highly functional and mechanical material for plastic film.
The proton exchange membrane belongs to the solid-state electrolyte. Although it is different from the aqueous electrolyte in the voltaic cell, it also has functions similar to the electrolyte solution which can pass positive and negative ions, as it is a conductor. The main function of the proton exchange membrane is to transmit protons, and such polymers in the fuel cell are the most important elements, and directly affect the performance and life of the fuel cell. The requirements of the film are as follows: (a) a good proton transmission capacity (high ion exchange capacity, a uniform microphase separation); (2) able to be isolated from the fuel (gas or methanol) contacts; (3) the catalyst layer in close contact with the film (a good adhesion); and (4) have sufficient mechanical strength and heat tolerance.
The current development of solid-state polymer electrolytes is listed as follows:
1. Perfluorinated Polymer:
PFSA, the proton exchange membrane which is sold commercially (Nafion; DuPont), the series model represents the differences in thickness. Since PFSA has a structure similar to that of the main chain of Teflon, it can provide good mechanical properties, making its life span up to 60,000 hours, and its proton conductivity is up to 0.10 S/cm. The diffusion of methanol in such films still has a serious impact; cells short circuit easily. In addition, the expensive price and high-temperature intolerance are also its shortcomings, and the material still has many possibilities for improvement. Moreover, when the ion exchange capacity (IEC) is high, it means that the concentration of the sulfonic acid (SO3H−) is high, too. The proton channel is increased, so the conductivity is also increased. However, the drawback is that the concentration of the sulfonic acid in the polymer film will directly affect the nature of the film. It is possible to convert the film into a hydrophilic film to cause a hard formation of the film and easy hydrolysis.
2. PFSA Material Modification:
The commercial Nafion is modified to reduce the diffusion of methanol, which can be used as a proton exchange membrane in the methanol fuel cell.
3. Andoxo-Acids Membrane:
A polymer with basic groups (such as ether, alcohol, imine, amide imide group) is added to a strong acid (such as sulfuric acid or phosphoric acid) to form a complex, wherein the acids and bases form hydrogen bonds. This type of proton exchange membrane has a high thermal stability, proton conductivity, mechanical strength, and flexible characteristics, and it is mainly used for high temperature proton exchange membranes. The film theoretically has an excellent nature at a low degree of acidification, but the acidification concentration is increased for high proton conductivity so that the nature is damaged. Thus, the nature of the film and degree of acidification should be balanced.
4. Hydrocarbon Polymer:
The chemical stability and thermal stability of this type of polymer are not as good as Nafion, but the film made of the polymer is cheap. Thus, the disadvantage can be improved upon by the molecular structure and the film can be formed by a normal process.
5. Organic/inorganic hybrid polymer films: Organic/inorganic compounds, for example, amorphous silica modified organic polymer film, are used to achieve the object of high temperature operation, improve the disadvantage of methanol permeability, and promote thermal stability without degrading proton conductivity. The addition of a hydrophilic inorganic compound can increase the capacity of moisture maintenance of the anode. The addition of a conductive inorganic compound can improve proton conductivity. Because an inorganic compound is crystalline, an inorganic film has poor mechanical properties. Therefore, an inorganic compound serves as the modifier. First, an organic polymer with good properties is produced, then the polymer would be modified by an inorganic compound to form an organic/inorganic hybrid polymer film applied to the methanol fuel cell. However, the mixing ratio thereof is still a problem for the purpose of use.
Please refer to FIGS. 1A, 1B, 1C, and 1D, which are schematic diagrams for showing the possible structures of the current sulfonated polymer used in a proton exchange membrane, in which the hatched segments represent the hydrophobic segments, and the unmarked segment represents the hydrophilic segments.
The sulfonated polymer shown in FIG. 1A has a linear long chain structure. Although it can be attached with a plurality of sulfonic acid groups, the sulfonic acid groups are not enough. The sulfonated polymer shown in FIG. 1B has a branched main chain structure. Although it can be attached with a plurality of sulfonic acid groups, the proton conductivity thereof is low at high temperatures. The sulfonated polymer shown in FIG. 1C has a linear long chain structure. Although it has more sulfonic acid groups and higher IEC, the mechanical properties and thermal stability are on the low side. The sulfonated polymer shown in FIG. 1D has a linear main chain structure and has a partially dense distribution of the sulfonic acid groups. Therefore, it has relatively better physical and chemical properties required for a proton exchange membrane.
Therefore, if a polymer of sulfonated poly(arylene ether)s and a manufacturing method thereof can be provided using highly steric monomer as a hydrophilic segment, more groups thereof would connect with more sulfonic acid groups. The thermal stability and the glass transition temperature of the polymer of poly(arylene ether)s would be enhanced and a film of the PAE would have good water uptake, size stability, oxidation stability, chemical resistance, mechanical properties, and process stability as well as low-cost production to thus solve the problems and technical issues in the structure and production of the above conventional sulfonated polymers.